1. Field of the Invention
The present invention relates generally to an ink-jet printing apparatus for performing predetermined printing operation by ejecting an ink toward a various printing medium such as paper, cloth, non-woven fabric, OHP sheet and so forth, and more particularly to an ink-jet printing apparatus for performing printing operation employing an ink containing a coloring agent which is water insoluble or has low solubility to water.
2. Description of the Related Art
As typical method for performing printing on cloth, wallcovering and so forth, a screen textile printing method to perform direct printing on cloth and so forth by employing a silk screen printing plate. In this method, with respect to an original image, at first, the silk screen printing plates are prepared for respective colors used in the original image. Then, the silk screen printing plate is loaded on a screen textile printing apparatus to perform printing by directly transferring the ink to the cloth or so forth through mesh of the silk screen printing plate.
However, in such screen textile printing method, a huge amount of process steps and working days are required for preliminarily preparing the silk screen printing plates. Further operation is required for blending of inks for respective colors, and registering of the silk screen printing plate per each color. In addition, since the printing apparatus per se is bulky, and the size of the apparatus is increased in proportion to number of colors to be used to require substantial space for installation. Also, a space for storing the silk screen printing plates become necessary Therefore, it has been proposed an ink-jet printing method to perform printing directly on a printing medium, such as the cloth, wallcovering and so forth. The ink-jet printing method is to eject fine ink droplet through ejection ports of the ink-jet head for performing printing image or so forth by forming ink dots on the printing medium such as the cloth, which does not require the screen printing plate which has been required in the conventional screen textile printing to significantly shorten the process steps and working days for forming the image on the cloth. Furthermore, the ink-jet textile printing method is advantageous for capability of down-sizing of the apparatus. In addition, since printing information for printing can be stored in various storage medium, such as tape, flexible disk, optical disk and so forth, the ink-jet textile printing system is superior in safekeeping and storage of the printing information. Furthermore, the ink-jet textile printing method is advantageous in easiness of processing of the printing information, such as changing of color, layout and so forth, of expansion and contraction of the image and so forth.
On the other hand, cloth as printing medium to be employed in ink-jet textile printing, extends in wide variety, such as natural fibers, e.g. cotton, silk, wool and so forth, synthetic fibers, e.g. nylon, rayon, polyester and so forth, mixed fabric of these fibers and so forth. Accordingly, in order to satisfactorily perform printing for cloth consisted of such wide variety of fibers, it is desirable to adapt dyes of the ink to the material fibers. For example, disperse dye is preferred for polyester fiber, metal complex salt dye is preferred for wool, vat dye or pigment is preferred for cotton. Amongst, disperse dye, metal complex dye, vat dye and pigment are known as water insoluble coloring agent or coloring agent having low solubility.
In order to prepare water based ink employing water insoluble dye or dye having low solubility, it is typically performed to prepare fine particle of material of dye and disperse the fine particle dye material into water by dispersing agent for emulsification. However, when the water based ink, in which the dispersing agent is dispersed and emulsified, coagulation and settling out of the dye in the ink can occur with time to cause variation of dye density resulting in fluctuation of printing density.
With respect to such problem of degradation of the printing quality, there has been proposed in Japanese Patent Application Laid-open No. 57342/1986 to provide a function of stirring the ink in an ink transporting passage from an ink storage portion to an ink ejection port of an ink-jet head.
However, the construction of the prior art proposed in the above-identified publication can be insufficient for satisfactorily using the water based ink employing the water insoluble dye or dye having low solubility.
Namely, fluctuation of density of the coloring agent due to coagulation and so forth is significant in the ink storage portion having relatively large ink storage capacity, in the ink transporting passage. Thus, it is typical to provide the ink stirring function in the ink storage portion. However, coagulation of the coloring agent and so forth may be caused in other portions, such as in a tube to be normally used as supply passage for the ink. Difference of density due to coagulation of the coloring agent or so forth should cause difference of specific gravity which causes motion depending upon position of the vertical position of the tube to further increase density difference. On the other hand, rubber and resin type tubes are frequently employed as the supply passage of the ink. Such tubes have a tendency to cause settlement or absorption of the coloring agent. It is considered that owing to property of the material of the tube, the coloring agent tends to be absorbed on a wall within the tube. When such tube is employed, particularly, in case of not used for a long period, even with small difference of positional relationship between vertical direction, motion of the coloring agent is caused to make density difference significant.
FIGS. 1A to 1D are diagrammatic views for explaining fluctuation of density in the tube, respectively.
Water based ink consisted of water insoluble coloring agent or coloring agent having low solubility, filled in the tube, has uniform density as initially filled, as shown in FIG. 1A. In the condition of being left in non-use, such as not performing printing, coagulation and settlement may be caused in the ink during this period to cause small density difference as shown in FIG. 1B.
On the other hand, if such small density distribution is caused in the portion of the tube having height difference as shown in FIG. 1C, the portion of the ink having low specific gravity moves upwardly and the portion of the ink having high specific gravity moves downwardly as shown in FIG. 1D. By this movement, density difference within the tube is promoted.
However, despite of the fact of presence of density fluctuation, the ink stirring function is typically provided in the ink storage portion in the view point of installation space. Accordingly, it has been difficult to solve the problem of fluctuation of the density of the coloring agent by stirring in the portion having small space, such as the tube.
On the other hand, in the ink-jet printing apparatus to be employed in printing press, textile printing and so forth, ink consuming amount is relatively large and ink consuming speed in relatively high. In case of such apparatus, it has been known to externally provide large capacity ink tank. When such construction is taken, the tube as the ink supply passage between the ink tank and the printing head becomes relatively long. Therefore, the ink amount in the tube is larger than that in the normal printer. On the other hand, it is practically not possible to maintain the ink supply passage completely horizontal. Therefore, it is inevitable to cause height difference in the tube positions. Accordingly, influence of the fluctuation of the density of the coloring agent caused in the tube for the printing density becomes significant in the ink-jet textile printing apparatus.